Abstract

A set of different samples of semi-insulating GaAs:Cr has been assessed independently for their Cr content using EPR measurements, and for their electrically active centres derived from experiments combining Hall, DLTS and optical absorption measurements. In the dark, only Cr2+ centres were observed by EPR whereas Cr3+ signals were always negligible and Cr1+ signals could never be observed. The Cr2+ concentration increases with the residual concentration of shallow donors (ND¯NA) and of a deep donor, probably the ‘pseudo-oxygen’ defect or EL2, thus indicating that compensation occurs by trapping of free electrons on the Cr3+ ions. The residual concentration of EL2 is found to be of the order of 1016 cm-3, in agreement with former evaluations. Furthermore, at 4.2 K the EL2 level lies above the Cr acceptor level.